Angry Birds and Facebook were the most downloaded iPhone apps of 2011, according to Apple. It thus shouldn't be too big of a shocker to learn Facebook was also the top app in the social networking category.

Craig Palli, vice president at marketing company Fiksu, told Reuters last year's top apps follow three main trends: the ability to connect with friends, discovery, and games that let users past the time and pick up where they left off.

Facebook for Android still has fewer monthly active users (MAU) than Facebook for iPhone. On the flipside, this means a larger percentage of Android users are returning to use the app daily compared to iPhone users. In other words, of the Android and iPhone users who have the official Facebook app, the former uses it more frequently than the latter.

The iPhone was first released in July 2008. More than a year later, the Android app followed with its first version in September 2009. Ever since, Android has been slowly trying to catch the iPhone version, but it's taken a while: over two years.

Unlike other platforms, Facebook develops the apps for iOS and Android. For iOS, Facebook has long-supported the iPhone and iPod touch, and only started supporting the iPad three months ago. For Android, Facebook is currently only supporting smartphones and has said nothing about tablets.